This disclosure relates to the field of marine well drilling. More specifically the disclosure relates to systems for marine well drilling having a pump in a drilling fluid return line to add energy to drilling fluid returning from a well to a drilling platform.
Marine drilling from a platform above the surface of a body of water may include drilling a “surface” section of a wellbore to a selected depth in formations below the water bottom. A “surface casing” or conduit may then be inserted into the surface section of the wellbore and cemented in place. After the surface casing is set in place, a well pressure control apparatus, such as a subsea “blowout preventer” (BOP) with a lower marine riser package (LMRP) may be coupled to the upper end of the surface casing, which is usually located proximate the water bottom. After the BOP/LMRP are assembled to the surface casing, a marine drilling riser is assembled on the platform and is ultimately coupled to the LMRP and extends therefrom to the platform proximate the surface of the body of water. There are other ways to assemble the foregoing components. In another embodiment, the LMRP is assembled to the bottom of and forms part of the marine drilling riser and an emergency disconnect, The LMRP may be disconnected from the BOP. In deep water the BOP is normally run suspended at the bottom of the LMRP when the riser is run in order to save the time required to “trip” the BOP into the body of water separately from the riser.
The marine drilling riser may be assembled from segments of conduit having flanges coupled to the longitudinal ends of each conduit segment. Assembly of the riser may include bolting the flanges together end to end until the required length of riser is formed. The riser in some embodiments may be a relatively large diameter conduit, e.g., between 16 and 30 inches in diameter to enable free passage therethrough of various drilling tools used to extend, by drilling, the length of the wellbore below the bottom of the surface casing.
Marine drilling riser may comprise one or more fluid lines extending outside of and generally parallel to the marine drilling riser. Such fluid lines may include, e.g., a choke line, a kill line and a booster line. The choke line and kill line may have fluid connections to a point below the BOP so that fluid pressure in the wellbore may be adjusted and/or controlled by pumping fluid into the kill line and/or controlling fluid discharge from the wellbore through the choke line. The choke line may have a controllable flow restriction (e.g., a variable orifice choke) disposed at the end of the choke line proximate or on the platform. The booster line may be used to pump additional drilling fluid into an annular space between the interior of the riser and the exterior of the drilling tools (e.g., drill pipe) extending through the riser so as to increase velocity of drilling fluid returning from the wellbore. The increased velocity may be required in some circumstances to lift drill cuttings from the wellbore below the BOP to the platform through the riser; the velocity of the returning drilling fluid may in some cases drop below that required to lift cuttings as the returning drilling fluid enters the marine drilling riser because of its relatively large diameter.
Marine drilling systems that provide a pump for returning drilling fluid to the platform are described, for example in U.S. Pat. No. 4,291,772 issued to Beynet and U.S. Pat. No. 6,454,022 issued to Sangesland et al. In the foregoing patents, the riser is described as being hydraulically opened to the wellbore below. In order to maintain a hydrostatic pressure in the wellbore annulus that is lower than would be provided if the entire length of the marine drilling riser were filled with drilling fluid of the same density as that pumped from the platform into the drilling tools, the riser may be partially or totally filled with sea water or air. As the drilling fluid leaves the wellbore annulus (the space between the drill pipe and the wellbore wall), it is diverted, through suitable valves to a line connected to the inlet of a pump (called a mudlift pump) that lifts the drilling fluid to the surface through a separate fluid return line. Typically, the mudlift pump is operated so that the interface between the drilling mud and the water or air column above in the riser is maintained at a selected depth level. Maintaining the selected level causes a selected hydrostatic pressure to be maintained in the wellbore.
There is a significant difference between the density of seawater and air or gas. As an example; if the entire effect of water depth is to be eliminated, and the riser is seawater filled, the drilling fluid/seawater interface may be placed close to seafloor and riser boost is not needed. If air or gas is used above the drilling fluid level in the riser, the interface is typically shallower than a drilling fluid/seawater interface for the same drilling fluid density in order to exert the same bottom hole pressure (BHP) in the well below the water bottom. In this case riser boost is needed in order to avoid cuttings build up in the drilling riser due to the large diameter and corresponding low fluid velocity if additional flow into the riser is not provided. The riser boost flow may in many cases be more than the drilling fluid circulation rate through the wellbore and thereby may comprise more than 50% of the rated flow for the mudlift pump, depending on the rated flow capacity of the mudlift pump.
In systems such as described in the foregoing two patents, the mudlift pump is located either proximate or just above the BOP and LMRP and is connected at its inlet from a fluid outlet on the marine drilling riser proximate the inlet of the mudlift pump. In the Sangesland et al. '022 patent, the fluid outlet from the riser is elevated a substantial distance from the BOP/LMRP and the mudlift pump inlet is proximate to the fluid outlet on the marine drilling riser.
For the apparatus described in the Sangesland '022 patent there is a minimum depth at where the mudlift pump can be placed in relation to its suction pressure; the hydrostatic pressure of the mud column inside the drilling riser needs to be greater than the frictional losses through the pump suction line. It would typically be better to position the mudlift pump somewhat deeper in order provide higher suction pressure/better margin. The differential pressure, pump head and pump horsepower required will remain the same as these parameters depend on the drilling fluid density (mud weight), drilling fluid flow rate and the riser fluid interface level from the mud level inside the drilling riser that is to be lifted back to surface. Elevating the pump as disclosed in the Sangesland patent will for the most part still require riser boost flow in order to transport drill cuttings up the riser annulus. The riser boost flow will then need to be lifted back to surface using the mudlift pump in addition to lifting the flow of drilling fluid that is pumped into the well through the drill pipe. This will necessitate larger pump size and horsepower as compared to the system disclosed in the Beynet '772 patent for any specific mud weight, drilling fluid circulation rate through the well (open hole) and bottom hole pressure.
By way of example, mudlift systems known in the art may have one or more of the following limitations. Drilling fluid return flow in the marine drilling riser needs boost flow to ensure proper drill cuttings transport. The boost flow requirement is a factor that determines the boost (e.g., mudlift) pump size. If a separate mud return line is used, for example as shown in the foregoing two patents to Beynet and Sangesland et al, such lines need to be installed onto the exterior of the riser in the moon pool or similar opening through the hull of a drilling platform. The moon pool may already be congested by reason of the riser and external lines being installed thereon. Thus a separate mud return line may require substantial rig and riser modifications. Such modifications may increase riser assembly time and installation cost.
In a controlled mud level system, e.g., as described in the Sangesland et al. patent, the formula for mudlift pump horsepower can be simplified as follows:N=f(Q*p*C)
where; N=pump horsepower (hp); Q=return fluid flow rate (gallons per minute—gpm); Δp=boost pump head (in psi) and C is a proportionality constant. Variations in pipe pressure losses have been ignored in the above expression as they will not be significant for this comparison.
The total flow through the drilling riser to be able to carry drill cuttings to surface will typically be on the order of 1800-2000 gpm. For a 17½ inch diameter drilled wellbore the flow will typically be 1200-1600 gpm. For a 12¼ inch diameter drilled wellbore the flow will be 700-1000 gpm, and for an 8½ inch diameter wellbore the flow will be about 400-600 gpm. The increase in flow required by using riser boost may be observed in Table 1.
TABLE 1Well diameterFlow Rate OpenRiser Flow(inch)Hole (gpm)(incl boost)Boost % of Flow17½1200-16001800-200020-3512¼ 700-10001800-200050-60 8½400-6001800-200070-80
It is desirable to reduce or eliminate the need for riser boost flow without the need for installation of a separate mud return line.